EP1474887B1 - Apparatus and method for determining combiner weights and log-likelihood ratios for symbols transmitted in a wireless communication system - Google Patents

Apparatus and method for determining combiner weights and log-likelihood ratios for symbols transmitted in a wireless communication system Download PDF

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Publication number
EP1474887B1
EP1474887B1 EP03729592A EP03729592A EP1474887B1 EP 1474887 B1 EP1474887 B1 EP 1474887B1 EP 03729592 A EP03729592 A EP 03729592A EP 03729592 A EP03729592 A EP 03729592A EP 1474887 B1 EP1474887 B1 EP 1474887B1
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Prior art keywords
symbols
gain vector
pilot
code
code symbols
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German (de)
English (en)
French (fr)
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EP1474887A1 (en
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Srikant Jayaraman
Ivan Jesus Fernandez Corbaton
John E. Smee
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Qualcomm Inc
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Qualcomm Inc
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7097—Interference-related aspects
    • H04B1/711—Interference-related aspects the interference being multi-path interference
    • H04B1/7115—Constructive combining of multi-path signals, i.e. RAKE receivers
    • H04B1/712—Weighting of fingers for combining, e.g. amplitude control or phase rotation using an inner loop
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7097—Interference-related aspects
    • H04B1/711—Interference-related aspects the interference being multi-path interference
    • H04B1/7115—Constructive combining of multi-path signals, i.e. RAKE receivers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
    • H04L1/0045—Arrangements at the receiver end
    • H04L1/0047—Decoding adapted to other signal detection operation
    • H04L1/005—Iterative decoding, including iteration between signal detection and decoding operation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
    • H04L1/0045—Arrangements at the receiver end
    • H04L1/0054—Maximum-likelihood or sequential decoding, e.g. Viterbi, Fano, ZJ algorithms
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7097—Interference-related aspects
    • H04B1/7103—Interference-related aspects the interference being multiple access interference
    • H04B1/7105—Joint detection techniques, e.g. linear detectors
    • H04B1/71057—Joint detection techniques, e.g. linear detectors using maximum-likelihood sequence estimation [MLSE]

Definitions

  • the present invention relates generally to wireless communications. More specifically, the present invention relates to recovering information bits from code symbols and pilot symbols transmitted over diversity channels.
  • FDMA frequency division multiple access
  • TDMA time division multiple access
  • CDMA code division multiple access
  • various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95).
  • AMPS Advanced Mobile Phone Service
  • GSM Global System for Mobile
  • IS-95A IS-95A
  • IS-95B IS-95B
  • ANSI J-STD-008 often referred to collectively herein as IS-95
  • proposed high-data-rate systems for data, etc. are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
  • Telecommunication Industry Association Telecommunication Industry Association
  • Communication systems configured in accordance with the IS-95 standard employ CDMA techniques to provide efficient and robust service.
  • Exemplary communication systems configured substantially in accordance with the use of the IS-95 standard are described in U.S. Patent Nos. 5,103,459 and 4,901,307 , which are assigned to the assignee of the present invention.
  • An exemplary system utilizing CDMA techniques is the cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate submission (referred to herein as cdma2000), issued by the TIA.
  • RTT Radio Transmission Technology
  • CDMA standard is the W-CDMA standard, as embodied in 3 rd Generation Partnership Project "3GPP" , Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214.
  • a known pilot signal is transmitted along with a data-bearing signal so that a receiver can coherently demodulate the data-bearing signal and recover the transmitted data.
  • Typical coherent receivers extract information about the wireless channel's amplitude and phase characteristics from the received pilot signal, and then use this extracted information for coherent demodulation of the data-bearing signal.
  • An other receiver such as described in the article " Joint Blind Rate Detection And Data Decoding Scheme for CDMA Mobile Stations In Frequency Selective Rayleigh Fading Channels" by G. Yang and S. Kallel, 2000 IEEE International Conference Personal Wireless Communication 17-20th Dec 2000, pages 449-453 , extracts information about the data rate used by the transmitter based on the data-bearing signal.
  • pilot and code symbols are used to determine optimized diversity combiner weights, which are then used by the receiver to determine the transmitted code symbols.
  • the optimized diversity combiner weights are used by the receiver to determine likelihood ratios for the received code symbols, which can then be used for soft-decision decoding of the transmitted data bits.
  • an apparatus for determining likelihood values of input data bits from a plurality of code symbols and a plurality of pilot symbols, the apparatus comprising: a memory element; and a processor configured to execute a set of instructions stored in the memory element, the set of instructions when executed on said processor performing the following method steps: determining a gain vector relating the plurality of code symbols and the plurality of pilot symbols in accordance with channel characteristics; and using the gain vector to determine likelihood values of a designated code symbol, wherein the input data bits are carried by the designated code symbol.
  • FIG. 1A is a block diagram of an encoding process.
  • FIG. 1B is a block diagram of a modulation scheme.
  • FIG. 2 is a block diagram of a receiver with a searcher and RAKE processor.
  • FIG. 3 is a flow chart illustrating a LLR computation procedure.
  • FIG. 4 is a flow chart illustrating a simplified LLR computation procedure.
  • FIG. 5 is a flow chart illustrating an implementation of the simplified LLR computation procedure.
  • the transmitted signals in communication systems are inherently prone to degradations such as channel noise and interference.
  • the transmitted data may or may not be recoverable at the receiver.
  • ECC Error Control Coding
  • the basic idea behind such techniques is to introduce redundant information in the stream of transmitted data. If errors were to occur in the reception of the transmitted signal, the data may still be recovered by exploiting this redundancy.
  • An example of an ECC technique is convolutional coding.
  • convolutional coding binary data bits are input to a finite state machine (FSM), which produces one or more binary outputs for every input data bit.
  • the outputs of this FSM are called code symbols.
  • a typical method for constructing such an FSM is through one or more convolutional encoders, i.e., finite impulse response (FIR) binary digital filters operating using arithmetic in the Galois Field GF(2). If the code symbols are corrupted by noise and interference during transmission over a noisy channel, the data bits may still be recoverable through suitable inferences based upon the corrupted code symbols.
  • FIR finite impulse response
  • the code symbols are "redundant", i.e., the code symbols contain information about not only the input data bits but also the "intemal state" of the FSM.
  • Methods for optimally inferring the input data bits from the received code symbols are known in the art and are commonly referred to as Trellis Decoding Algorithms, e.g., the Viterbi Algorithm, or the Stack Algorithm.
  • Turbo coding employs two or more convolutional encoders in parallel, in series, or in a combination thereof.
  • the resulting sequence of code symbols also possesses redundant information about the input data bits.
  • methods for optimally inferring the input data bits from the received code symbols are known in the art and are commonly referred to as turbo decoding algorithms.
  • a "source” In a typical communication system, a "source” generates a stream of information bits representing, for example, voice or data "traffic". This stream of bits is subdivided and grouped, various control bits are appended, and the result is packed into a suitable format for transmission. Voice and data traffic can be transmitted in various formats, such as, e.g. frames, packets, and subpackets.
  • the scope of the embodiments described herein extends to all wireless communication systems using any of the various transmission formats. However, for the purpose of illustrative ease, the term "frame” will be used herein to describe the transmission format in which traffic is carried.
  • FIG. 1A is a functional block diagram of an encoding process.
  • Information bits ⁇ b 1 , . . ., b n ⁇ are encoded at block 100.
  • a repeating element is represented at block 110, where the encoded bits are repeated at a predetermined repetition rate.
  • the encoded and repeated symbols are then punctured at block 120.
  • the rates at which encoding, repeating, and puncturing occur are system-defined parameters that rely upon transmission rate requirements.
  • Block 130 represents the interleaving process.
  • the output of the interleaver is referred to herein as modulation symbols.
  • FIG. 1B is a functional block diagram of this process.
  • the modulation symbols ⁇ d ⁇ (1), . . ., d ⁇ (N') ⁇ that are the output of the interleaver at block 130 undergo orthogonal spreading by a Walsh covering element 140a.
  • a pilot sequence is concurrently spread by Walsh covering element 140b.
  • the outputs from blocks 140a and 140b are then combined at summation element block 150 and then spread in quadrature at block 160.
  • the resulting stream is PN-spread.
  • the resulting stream is baseband filtered and modulated onto a transmit carrier signal.
  • the signal propagates to the receiver over multiple transmission paths and is received as a superposition of multiple components, each with its own amplitude, phase and time delay. These multiple transmission paths are referred to as “multipaths" and are commonly caused by reflections off objects present in the transmission path.
  • multipaths multiple transmission paths
  • the searcher usually determines the time delays of the multipath components in the received signal.
  • the RAKE processor comprises multiple "finger", each of which is synchronized to the time delay of a particular multipath component.
  • Each RAKE finger is configured to PN-despread the sampled and digitized waveform using the PN code synchronized to that finger's particular time delay. Additionally, each RAKE finger is able to perform Walsh de-covering to separate the modulation symbols from the orthogonal pilot symbols.
  • the output of the RAKE processor can be represented by a sequence of received modulation symbols ⁇ x 1 , ... , x N' ⁇ , and a corresponding sequence of pilot symbols ⁇ y 1 , ... , y N' ⁇ .
  • FIG. 2 is a block diagram of the above-described process within a receiver. A signal is received over the air by at least one antenna 200.
  • the signal is match-filtered, sampled, digitized, and down-converted to complex baseband by a pre-processing element 210 before the signal is fed to the searcher 220 and the RAKE processor 230.
  • the searcher 220 determines the time delays of each of the components of the received signal and assigns a RAKE finger (not shown) in the RAKE processor 230 to each component.
  • the outputs of the RAKE processor are the two sequences ⁇ xj ⁇ and ⁇ yj ⁇ .
  • the received modulation and pilot symbols are complex vectors of length L , where L is the number of fingers in the RAKE processor.
  • the I th component of the vector x j is the J th modulation symbol received on the I th RAKE finger.
  • the I th component of the vector y i is the J th pilot symbol received on the I th RAKE finger.
  • the vectors are complex because the quantities represented are in complex baseband form.
  • n j and ⁇ j denote the noise and interference affecting the received modulation and pilot symbols, respectively.
  • Both n j and ⁇ j can be modeled as sequences of independent, L -dimensional, random vectors.
  • the components of n j and ⁇ j have variance ⁇ t 2 and ⁇ p 2 , respectively.
  • the multipath gain vector ⁇ j is an L-dimensional vector whose I th component denotes the complex amplitude and phase gain for the I th RAKE finger and the J th received symbol.
  • the multipath gain vector ⁇ is a function of the multipath channel and is a priori unknown to the receiver.
  • this representation can arise from assigning RAKE fingers to different multipath components on different carriers. This method is called frequency or carrier diversity.
  • the goal at the receiver is to recover the information bits ⁇ b 1 , ..., b n ⁇ from the received modulation and pilot symbols, ⁇ x 1 , ... , x N' ⁇ , and ⁇ y 1 , ... , y N' ⁇ . This, in turn, requires decoding the convolutional or turbo code.
  • LLRs Log Likelihood Ratios
  • LLRs ⁇ ( d j ) ⁇ are known in the art as "sufficient statistics" for recovering the transmitted information bits.
  • the LLRs contain all the information in the received symbols that is relevant to decoding the frame.
  • typical convolutional decoding algorithms such as, e.g., the Viterbi Algorithm or the Stack Algorithm, require code symbol LLRs as their inputs.
  • Turbo decoding algorithms such as, e.g., those based on the Bahl-Cocke-Jelinek-Raviv Algorithm, also require code symbol LLRs as their inputs.
  • pilot Filter MRC code symbol LLRs are computed based on channel characteristic information derived solely from the received pilot symbols.
  • the pilot signal is transmitted concurrently with the data-bearing signals.
  • the transmission paths of the pilot signal share similar amplitude and phase characteristics as the transmission paths of the data-bearing signals.
  • the practice of those of ordinary skill in the art is to use information derived from observations of the pilot signal to determine likelihood values for the code symbols of the data-bearing signal.
  • this combiner weight y is close to ⁇ , which is the true multipath gain vector, the performance approaches nearly ideal maximal ratio combining (MRC).
  • MRC maximal ratio combining
  • the pilot filter weight y usually deviates from the true multipath gain vector because the pilot channel SNR is not high and because there are only finitely many pilot symbols N in the estimate. The deviation from the ideal can be quite large when the pilot channel SNR is "weak".
  • N the number of pilot symbols in the estimate, yields a more accurate combiner weight when the channel is static.
  • increasing N may affect overall performance adversely because the channel's characteristics are no longer fixed. In this case the changing channel characteristics outweigh any increased benefit from a longer filter length.
  • the embodiments described herein lead to more accurate combiner weights by exploiting the fact that the received modulation symbols also carry information on the multipath gain vector ⁇ . This information can be used to improve the determination of the LLRs of the code symbols, and consequently, the performance of the decoder. The improved determination of the combiner weight occurs without sacrificing overall performance on slowly time-varying channels.
  • the embodiments herein describe a new methodology for determining code symbol LLRs, which results in a simple method for the computation of said LLRs. Using these embodiments, a SNR gain on the order of 0.5 dB over the traditional pilot filter MRC approach can be achieved.
  • M , M , N , N depends on the rate of variation of the channel and the desired tradeoff between computational complexity and decoder performance. Examples of different parameter value implementations are discussed below.
  • the indices j and j' can range from negative infinity to positive infinity.
  • the embodiments described herein are for determining the LLR for d ⁇ ( k ), where ⁇ is a one-to-one and onto mapping of the integers, i.e., a permutation.
  • J ⁇ j k - M ⁇ ⁇ k + M ⁇ and J ⁇ ⁇ j ⁇ : k - N ⁇ ⁇ j ⁇ ⁇ k + N ⁇ ,
  • d ⁇ k + 1 , d ⁇ j : j ⁇ J - k max ⁇ ⁇ , d ⁇ j : j ⁇ J - k ⁇ p ⁇ ⁇ ⁇ x j , y j ⁇ : j ⁇ J , j ⁇ ⁇ J ⁇
  • d ⁇ k - 1 , d ⁇ j : j ⁇ J - k
  • p ⁇ ⁇ ( ⁇ ⁇ ⁇ ) denotes the respective conditional density when the multipath gain vector is ⁇ .
  • the following definitions are also used: y ⁇ 1 N ⁇ j ⁇ ⁇ J ⁇ y j , where N ⁇
  • an iterative procedure can be used to determine ⁇ k .
  • ⁇ is a system defined tolerance value and the norm ⁇ • ⁇ can be any arbitrary mathematical function, such as the Euclidean distance, the sup norm, etc.
  • step 320 label the last value of ⁇ n as ⁇ + .
  • step 340 label the last value of ⁇ n as ⁇ - .
  • ⁇ k f k ⁇ ⁇ ⁇ - , - 1 - f k ⁇ ⁇ ⁇ + , + 1
  • FIG. 4 is a flowchart of the procedure to calculate ⁇ k .
  • ⁇ 0 y .
  • ⁇ p 2 / ⁇ t 2 which is a quantity sometimes known as the Pilot-to-Traffic ratio. This quantity is closely related to the ratio of the transmit gain of the pilot channel to the transmit gain of the traffic or data channel. Governing standard bodies mandate the value of this ratio in accordance with other transmission parameters. Also necessary for the embodiments are M , which is the number of modulation symbols, and N , which is the number of pilot symbols. The choice of M and N is flexible as long as the slowly time-varying model of Equation 5 is not violated.
  • the simplified LLR computation procedure is implemented in the context of a cdma2000 1 X reverse link.
  • Received modulation symbols and pilot symbols ⁇ x k ⁇ and ⁇ y k ⁇ are considered from a sequence of transmitted frames.
  • k - ⁇ , ..., ⁇ .
  • FIG. 5 illustrates the implementation of the simplified LLR computation procedure.
  • step 500 choose a value for K.
  • K that divides N' evenly.
  • step 510 divide the N' symbols in a frame into groups where the I th group contains symbols with indices iK + 1, ..., (i+1)K. There are N' / K such groups in a frame.
  • step 520 set a counter for i, ranging from 0 to N' / K -1.
  • Equation 9 iterate Equation 9 until ⁇ n - ⁇ n -1 ⁇ ⁇ or S iterations have occurred, whichever occurs first.
  • ⁇ p 2 / ⁇ t 2 is the Pilot-to-Traffic ratio, which is known. Let the last value of ⁇ n . be ⁇ .
  • ⁇ k 2 ⁇ t 2 ⁇ Re ⁇ ⁇ H x k .
  • step 570 increment i and repeat steps 530 - 560, until the entire set ⁇ 1 , . . ., ⁇ N' ⁇ is obtained.
  • step 580 deinterleave ⁇ 1 , . . . , ⁇ N' ⁇ and use a decoder to recover the information bits.
  • the apparatus required for the above-described embodiments comprises a receiver with a processor and memory element that are configured to perform the above calculations.
  • the same processor or another processor can perform the function of deinterleaving.
  • the functionality of the processor and memory element can be incorporated into the RAKE processor 230 of FIG. 2 or into another portion of the receiver, such as a decoder.
  • the decoder can be any standard decoder or specialty decoder.
  • PCG Power Control Group
  • K is selected to equal 1
  • the number of iterations S equals 5
  • ⁇ equals 0
  • ⁇ is computed separately for each of the 1536 code symbols, using a 2 PCG "symmetric window" of received pilot and code symbols. The number of iterations is fixed at 5 for each of the 1536 computations of ⁇ .
  • the embodiment can be implemented to use unequal numbers of code/modulation symbols and pilot symbols.
  • ⁇ is computed once per PCG-worth of code symbols. Hence, only 16 computations are required for each 20 ms frame.
  • g ⁇ (z) is a monotonic, non-decreasing function of z.
  • a different choice of g ⁇ (z) may help improve the convergence of the iterations.
  • the code symbols d ⁇ (J) and pilot symbols can be conceptualized as being transmitted on an L th -order diversity channel.
  • the outputs of the diversity channel are two complex vectors of length L corresponding to received code symbols and pilot symbols.
  • an L th -order diversity channel can be created conceptually by assigning L fingers to L different multipath components, at different time delays and different antennas in a RAKE receiver.
  • the embodiments above have been described in the context of a cdma2000 reverse link, the embodiments can be extended to other communication systems without undue experimentation. Furthermore, the embodiments can be extended for use on either the forward or the reverse links, and in particular, can be most useful whenever the pilot signal is weak.
  • DSP digital signal processor
  • ASIC application specific integrated circuit
  • FPGA field programmable gate array
  • a general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine.
  • a processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.
  • a software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art.
  • An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium.
  • the storage medium may be integral to the processor.
  • the processor and the storage medium may reside in an ASIC.
  • the ASIC may reside in a user terminal.
  • the processor and the storage medium may reside as discrete components in a user terminal.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Artificial Intelligence (AREA)
  • Radio Transmission System (AREA)
  • Mobile Radio Communication Systems (AREA)
  • Error Detection And Correction (AREA)
  • Detection And Prevention Of Errors In Transmission (AREA)
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EP03729592A 2002-01-15 2003-01-07 Apparatus and method for determining combiner weights and log-likelihood ratios for symbols transmitted in a wireless communication system Expired - Lifetime EP1474887B1 (en)

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US10/050,399 US6901103B2 (en) 2002-01-15 2002-01-15 Determining combiner weights and log likelihood ratios for symbols transmitted on diversity channels
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CN1640047A (zh) 2005-07-13
WO2003061180A1 (en) 2003-07-24
KR100956460B1 (ko) 2010-05-07
DE60325336D1 (de) 2009-01-29
JP4130632B2 (ja) 2008-08-06
HK1077432A1 (en) 2006-02-10
CA2473233A1 (en) 2003-07-24
TW200307411A (en) 2003-12-01
KR20040070313A (ko) 2004-08-06
MXPA04006829A (es) 2004-10-11
TWI319667B (en) 2010-01-11
JP2005515698A (ja) 2005-05-26
EP1474887A1 (en) 2004-11-10
AU2003216045A1 (en) 2003-07-30
BR0306895A (pt) 2004-12-14

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